1. Field of Endeavor
The present invention relates to a Rogowski coil assembly, in particular for use in a current measurement apparatus like a Rogowski sensor, in particular for measuring high frequency electrical currents.
2. Brief Description of the Related Art
Rogowski coils are well known in the art to provide a reliable device for measuring and sensing current flow at a given point in an electrical system.
Its functioning is based on the fact that a current flowing through a conductor generates a magnetic field that, in turn, induces a voltage in the coil. This is based upon Faraday's law stating that “the electromotive force induced in a close circuit is proportional to the time rate of change of the total magnetic flux linking the circuit”.
A Rogowski coil is generally fabricated from a conductor that is coiled or wound on a non magnetic core, which may be for example air or a non-magnetic material.
The coil may be placed or bent around a conductor whose currents are to be measured with a coil.
However, a physical problem arises when trying to use a long Rogowski coil for measuring and sensing currents in the MHz range where a large bandwidth is needed.
Indeed, the bandwidth of a Rogowski coil depends on the number of windings, the length and the cross section, whereas the sensitivity depends on the number of windings per length and the cross section.
Therefore, in some applications that need a long Rogowski coil with a large bandwidth, the number of windings per length or the cross section is reduced, leading thus to a lower sensitivity that is not acceptable for some applications.
Some specific attempts have been conducted to overcome this problem.
The document WO 0072027 discloses an electrical current sensor having a quite complex winding scheme in order to reduce the magnetic interference fields and thus achieve good sensitivity at high frequency.
The document WO 98 48287 proposes a device with a wide pass band for measuring electric current intensity in a conductor by using a specific combination of a Rogowski coil and several magnetic field detectors, as for example Hall sensors, where a processor combines all information of the measurement result in order to achieve a relevant response in the high frequency band.
However, due to their complexity and structure, these solutions are not satisfying.
In addition, they do not seem to be adapted when monitoring, for example, shaft currents of electrical machines for fault detection as described in U.S. Pat. No. 7,102,379. Indeed, when spark erosion occurs on the shaft, sharp spikes may be observed in the time signal of the shaft current within a frequency bandwidth of 1.5 MHz to 40 MHz, the shaft being of typically 0.6 to 1.5 m of diameter.